Lambda deployment has no instances. There's no concept of "spinning up a new server"; instead, there are versions of function code and Aliases that point to those versions. Blue/Green's "traffic shift" is implemented here as a change in Alias weighted routing.
This simplicity is its strength. Rollback is just reversing the Alias weight to the previous version — completed in seconds. There's no need to terminate new servers or reconfigure load balancers. But behind this simplicity lies a mathematical understanding of "exactly how much traffic goes to which version." That's the difference between Canary and Linear, and it's the most frequently confused topic on the exam.